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This Italian study lays out the issues of Radon and Thoron based radiation issues clearly. They lay out the strict correlation between Radon/Thoron and health risks while admiting that sometimes building material based Radon or Thoron are the most likely cause of high levels in a home.

Their materials in the study come from the Lazio region of Italy. Mostly igneous rocks, lava based, although they include some travertines. They point out that as much as 30% loss of result from leakage and back diffusion of the Radon. Back diffusion is where the Radon returns to the material instead of accumulating where it can be measured.

Another point is that moisture content actually raises the Radon release, the water inside the pores stops the Radon atom from recoiling into another crystal matrix inside the stone, stopping the Radon inside the pores and crevices where it can find its way out of the stone. Looking at Table 2, you can see the huge diference in Radon versus Thoron emitted in some stones, yet many of the Radon tests specifically omit any Thoron results as irrelevant. As much as 750 times more Thoron as Radon emitted.

The study has five conclusions, that building materials need to be classified by their Radon/Thoron exhalation rates, that leakage and back diffusion must be accounted for since it can reduce the measured levels by up to 30%, that the exhalation rates are dependent on grain size, humidity and temprature, that Thoron gas is a serious health concern and should not be dismissed, and that there should be catalogs of materials with Radon and Thoron exhalation rates so that builders could choose low level materials.